Fuel System Diagnosis

Fuel System Diagnosis

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle (Diagnostic System Check - Vehicle) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Strategy Based Diagnosis) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Diagnostic Procedure Instructions) provides an overview of each diagnostic category.

Circuit/System Description

The engine control module (ECM) supplies a high voltage signal to the fuel pump control module (FPCM) when the ECM detects that the ignition is ON. The high voltage signal from the ECM to the FPCM remains active for two seconds unless the engine is in Crank or Run. While this signal is being received, the FPCM closes the ground switch of the fuel pump, and also supplies a varying voltage to the fuel tank pump module in order to maintain the desired fuel rail pressure.

The fuel system is an electronic returnless on-demand design. A returnless fuel system reduces the internal temperature of the fuel tank by not returning hot fuel from the engine to the fuel tank. Reducing the internal temperature of the fuel tank results in lower evaporative emissions.

An electric turbine style fuel pump attaches to the primary fuel pump module inside the fuel tank. The fuel pump supplies fuel through the fuel feed pipe to the high pressure fuel pump. The high pressure fuel pump supplies fuel to a variable-pressure fuel rail. Fuel enters the combustion chamber through precision multi-hole fuel injectors. The high pressure fuel pump, fuel rail pressure, fuel injection timing, and injection duration are controlled by the ECM.

The primary fuel pump module also contains a primary jet pump and a secondary jet pump. Fuel pump flow loss, caused by vapor expulsion in the pump inlet chamber, is diverted to the primary jet pump and the secondary jet pump through a restrictive orifice located on the pump cover. The primary jet pump fills the reservoir of the primary fuel pump module. The secondary jet pump creates a venturi action which causes the fuel to be drawn from the secondary side of the fuel tank, through the fuel transfer pipe, to the primary side of the fuel tank.

Reference Information
Description and Operation

Fuel System Description (Fuel System Description)
Electrical Information Reference

* Circuit Testing (Circuit Testing)
* Connector Repairs (Connector Repairs)
* Testing for Intermittent Conditions and Poor Connections (Testing for Intermittent Conditions and Poor Connections)
* Wiring Repairs (Wiring Repairs)

Scan Tool Reference

Control Module References (Programming and Relearning) for scan tool information
Special Tools Required

* CH 48027 - Digital Pressure Gauge
* J 37287 - Fuel Line Shut-off Adapter

Circuit/System Verification

Note:
* Repair all fuel system related DTCs before performing this diagnostic.
* Inspect the fuel system for damage or external leaks before proceeding.
* Verify that adequate fuel is in the fuel tank before proceeding.
* The fuel pump may need to be commanded ON a few times in order to obtain the highest possible fuel pressure.
* Before proceeding with this test review the User Manual CH 48027-5 for Safety Information and Instructions.
* Do not perform the Fuel System Diagnosis if the ECT is above 60°C (150°F). High fuel pressure readings may result due to hot soak fuel boiling. With the engine OFF, the fuel pressure may increase to the setting of the pressure relief regulator valve, 580 kPa (84 psi).

1. Ignition OFF, all accessories OFF, install a CH 48027 - Gauge. Refer to Fuel Pressure Gauge Installation and Removal (Fuel Pressure Gauge Installation and Removal).
2. Ignition ON, engine OFF, command the fuel pump ON with a scan tool. Verify the fuel pressure is between 276-600 kPa (40-87 psi) and does not decrease more than 34 kPa (5 psi) in 1 min.
3. Engine idling at normal operating temperature, verify the fuel pressure is between 297-400 kPa (43-58 psi).

Circuit/System Testing

Note:
* The fuel pump may need to be commanded ON a few times in order to obtain the highest possible fuel pressure.
* DO NOT start the engine.

1. Ignition ON, engine OFF, command the fuel pump ON with a scan tool and observe the fuel pressure gauge while the fuel pump is operating. Verify the fuel pressure is between 276-600 kPa (40-87 psi).

• If the fuel pressure is less than the specified range, test, inspect, and repair the items listed below. If all items test normal, replace the primary fuel pump module.

* Restricted fuel feed pipe
* Inspect the harness connectors and the ground circuits of the fuel pump for poor connections.

2. Verify that the fuel pressure does not decrease more than 34 kPa (5 psi) in 1 min.

• If the fuel pressure decreases more than the specified value, perform the following procedure:

1. Ignition OFF, relieve the fuel pressure. Refer to Fuel Pressure Relief (Service and Repair).
2. Install the J 37287 - Adapter between the chassis fuel feed hose and the engine compartment fuel feed pipe.
3. Open the valve on the J 37287 - Adapter.
4. Ignition ON, command the fuel pump ON with a scan tool and bleed the air from the CH 48027 - Gauge.
5. Command the fuel pump ON and then OFF with a scan tool.
6. Close the valve on the J 37287 - Adapter.
7. Monitor the fuel pressure for 1 min.

• If the fuel pressure does not decrease more than 34 kPa (5 psi) within the specified time, replace the primary fuel pump module.

3. Relieve the fuel pressure to 69 kPa (10 psi). Verify that the fuel pressure does not decrease more than 14 kPa (2 psi) in 5 min.

• If the fuel pressure decreases more than the specified value, replace the primary fuel pump module.

4. Operate the vehicle within the conditions of the customer's concern while monitoring the fuel pressure with the CH 48027 - Gauge. The fuel pressure should not drop off during acceleration, cruise or hard cornering.

• If the fuel pressure drops off, test, inspect, and repair the items listed below. If all items test normal, replace the primary fuel pump module.

* Restricted fuel feed pipe
* Inspect the harness connectors and the ground circuits of the fuel pump for poor connections

5. If the fuel system tests normal, refer to Symptoms - Engine Controls (Symptoms - Engine Controls).

Repair Instructions

Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.

* Fuel Feed Pipe Replacement (LF1) (Fuel Feed Pipe Replacement)Fuel Feed Pipe Replacement (LLT) (Fuel Feed Pipe Replacement)
* Fuel Feed Rear Pipe Replacement (Fuel Feed Rear Pipe Replacement)
* Fuel Line Replacement - Chassis (LF1/LLT) (Service and Repair)
* Fuel Tank Fuel Pump Module Replacement - Right Side (Fuel Tank Fuel Pump Module Replacement - Right Side)